The present invention relates to T-circuits produced using microstrip technology and comprising a phase-shifting element that gives a given phase shift, the T-circuit operating in broadband.
The present invention applies in particular to the field of broadband antenna networks. In this type of network, the width of the frequency band is often limited by the bandwidth of the elemental radiating element and by the bandwidth of the supply network. This is particularly the case when use is made of a phase shift in the excitation of the radiating elements. This type of phase shift is used in particular when the radiating elements produced, for example using printed technology, are excited using the well-known technique of sequential rotation. For networks of radiating elements of the above type, the supply network is usually produced using microstrip technology and consists of at least one T-circuit connected via microstrip lines and bends to the various radiating elements. The supply network thus distributes the energy to each of the radiating elements. In order for these radiating elements to be excited with the desired phase, bits of line are added on one side of the T-circuit or circuits. However, this phase shift is valid only for a narrow frequency band.
The behavior of the micro strip lines of the T-circuits and of the bends is actually well known to those skilled in the art and provides an explanation for the operation over a narrow frequency band.
In the case of microstrip lines, a length of microstrip line introduces a phase shift "PHgr"=xcex2L where L is equal to the length of the line and xcex2 is the phase constant. In a known way, xcex2 depends on the substrate, on the frequency and on the width of the microstrip line. Its value is given by:
xcex2=2xcfx80/xcexg 
where xcexg=xcex0/xcex5reff, 
xcexg being the guided wavelength. 
In this formula, xcex5r is the effective dielectric constant and depends on the width of the line, on the height of the substrate on which the line is produced, on the thickness of the metallization, on the dielectric constant of the substrate and on the wavelength, and xcex0 is the wavelength in a vacuum (associated with the frequency). This therefore explains why the lines do not have the same phase for different frequencies.
As is known, a T-circuit like the one depicted in FIG. 1, has equivalent line lengths between port 1 and port 2 and between port 1 and port 3. As a result, the value Ang(S21)xe2x88x92Ang(S31)=0, irrespective of the working frequency.
In addition, in a supply network produced using microstrip technology, use is also made of bent lines which, among other things, allow for changes in direction so that energy can be supplied to the radiating element. In terms of phase shift, it is possible to find a length of bend equivalent to the length of a line. Thus, the phase shift of bend is equal to "PHgr"=xcex2bend xc3x97 Lbend,
where xcex2bend is the phase constant in the bend and
Lbend is the electrical length in the bend.
As depicted in Figure 2, T-circuits comprising a phase-shifting element have already been and in that the produced in the prior art. These circuits are based on the principle of a T-circuit with lines of identical length L2 on each side of the exit from the T and followed by bent lines comprising bits of line L1 of identical length. The circuit will display a phase difference Ang(S31)-Ang(S21)=0, regardless of the frequency, if the length of the lines between port 1 and port 2 and between port 1 and port 3 is the same. As a result, in order to introduce a phase shift of a given value, for example of 180xc2x0, between the exit ports 2 and 3, all that is required is for one of the lines to be lengthened by a length L such that PLxc3x97180xc2x0. This can be done using bits of line on each side of a bend, of a length such that "PHgr"xc3x97180xc2x0 and "PHgr"-1xc3x970 xc2x0, as depicted in FIG. 2. However, all of the simulations carried out on such a T-circuit show that this condition is valid only for the central frequency and that the phase shift of 180xc2x0 is no longer obtained when this central frequency is departed from.
Thus, the object of the present invention is therefore to propose a T-circuit produced using microstrip technology comprising a phase-shifting element such that the T-circuit can operate over a large frequency band.
In consequence, a subject of the present invention is a T-circuit produced using microstrip technology with two branches of identical length L2 comprising a phase-shifting element producing a given phase shift "PHgr" by extending one of the branches, the T-circuit operating in broadband, characterized in that it comprises at least one bend extending the branch without the phase-shifting element length L2 is equal to a multiple of xcexg/2 where xcexg is the guided wavelength.
In this case, the phase-shifting element is formed by a microstrip line of length L xc3x97"PHgr"/xcex2 where xcex2 is the phase constant, xcex2 being calculated as mentioned here in above. As a preference, the phase-shifting element is extended by a line element of length L71 xc3x97 L1 +Lbend and the bend is extended by a line element of length L1, these elements for example allowing connection to radiating elements.
According to another feature of the present invention, the phase-shifting element is formed of a bend of a length such that a phase shift of "PHgr"/2 is distributed on each side of the bent. In this case, each bent is extended by a line element of identical length L1 for connection, for example, to a radiating element.
The present invention also relates to a supply circuit for a broadband antenna network produced using microstrip technology, characterized in that it comprises at least one T-circuit exhibiting the characteristics described hereinabove.